The high affinity (strept)avidin-biotin binding system is widely utilized in pre-clinical diagnostic applications and is under evaluation as a molecular component for imaging and tumor-targeted cancer therapeutic {Rusckowski, 1996; van Gog, 1998}. It is believed that the biotinidase enzyme, which is present in serum and tissue of both animals and humans in nanomolar concentrations, cleave the biotinamide bond linking biotin (vitamin H) and lysine in biocytin, such that this essential vitamin can be recycled {Foulton, 1998; Hymes, 1996}. For in vivo studies, though, targeted biotin derivatives must have serum stability and aqueous solubility {Wilbur, 1997}. The nonspecific nature of the cleavage of biotinamide bonds in biotin conjugates has made it imperative that biotin derivatives employed in vivo be designed in a manner that blocks the enzyme activity.
The multimodality imaging of direct or pre-targeting (strept)avidin-biotin strategy has been elusive. The detection of biotin on molecules is facilitated greatly by the wide variety of (strept)avidin-based technologies exploiting the extremely strong noncovalent interaction of (strept)avidin for biotin, with a Ka of 1015 M. Usually in a pre-targeting strategy, a monoclonal antibody-(strept)avidin conjugate is injected into a patient, allowed to localize at the tumor over 24-48 h, followed by the clearing of excess reagent from blood and administration of the radiolabeled biotin derivative.